This invention relates to laser printing and, more particularly, to analog exposure control for selective speed xerographic printing.
Selective speed xerography was recently commercially introduced as a feature of the Telecopier 200 facsimile transceiver, which is manufactured and sold by Xerox Corporation. Those units provide an operator with the option of selecting a nominal transmission time of approximately two, three, four or six minutes for a standard 81/2".times.11" document. Furthermore, they rely on analog video signalling to convey the information content of a subject copy from a transmitting terminal to a receiving terminal at a rate consistent with the document transmission time selected.
When the Telecopier 200 transceiver is operating in its receive or print mode, a low power HeNe laser selectively discharges a more or less uniformly charged photoreceptor in response to the incoming video signal, thereby forming a latent electrostatic image of the subject copy. Generally standard techniques are then employed to develop or tone the latent image and to transfer and fuse the developed image onto a plain paper substrate.
One of the hurdles which had to be covercome to accommodate the different document transmission times is that the xerographic process is exposure time sensitive--viz., the shading of a xerographically produced image varies as a direct function of the exposure time, if all other parameters are held constant. The initial solution to that problem, which is disclosed and claimed in a commonly assigned Mason et al. U.S. Pat. No. 3,858,004 which issued Dec. 31, 1974, on a "Filter for Selective Speed Xerographic Printing in Facsimile Transceivers and the Like", involved precision optical attenuation of the laser beam to hold the radiant energy incident on the photoreceptor within a predetermined range of power per unit area, regardless of the document transmission time selected. Thus, several neutral density optical filters, each precisely matched to a different one of the document transmission times, were called for so that a filter providing the appropriate degree of optical attenuation for the document transmission time selected could be interposed between the laser and the photoreceptor in preparation for carrying out the printing process.
That has proven to be a reasonably effective solution to the problem, but experience has shown that it suffers from at least a couple of shortcomings. First, the precision required of the filters dictates the use of complex and sophisticated stacks of optical elements, which means that the filters are expensive and also liable, unless great care is taken, to introduce undesirable optical distortions. Secondly, the filters do not permit of the power per unit photoreceptor area for the different document transmission times being easily adjusted in the field, which means that fine tuning of the transceiver to the actual operating environment is inordinately difficult, if possible at all.